Conventionally, a radio terminal which performs real-time communication (such as VoIP) with a communication terminal (correspondent terminal) through a network is known (see Patent Document 1, for example). The radio terminal is provided with a buffer for temporarily storing packets to absorb a delay time of the network. The amount of packets stored in the buffer is determined according to the delay time of the network. Note that a delay time of a network is a concept which includes not only a time that a packet from the communication terminal (correspondent terminal) held in the network (holding time) but also variation (jitter) in the holding time.
In addition, an AJB (adaptive jitter buffer) control technique targeted for a single network is proposed in which a stored packet amount (jitter buffer size) is changed by controlling a reproduction rate according to a delay time of received packets (holding time and variation in the holding time). In the AJB control technique, the stored packet amount is reduced by changing the packet reproduction rate to a rate higher (faster) than a predetermined rate when the amount of packets stored in the buffer becomes larger than an optimal packet amount. On the other hand, the stored packet amount is increased by changing the packet reproduction rate to a rate lower (slower) than the predetermined rate when the amount of packets stored in the buffer becomes smaller than the optimal packet amount.